1. Field of the Invention
The invention generally relates to phase locking techniques and, in particular, relates to a phase-locked, voltage-controlled oscillator.
2. Background of the Prior Art
FIGS. 1 and 2 illustrate, in schematic form, the prior art techniques employed for phase locking and frequency changing of a voltage-controlled oscillator. FIG. 1 particularly illustrates the phase locking system as employed in the prior art. In general, the voltage-controlled oscillator 6 is phase locked to the oscillator 1 by the following sequence. If the frequency of the voltage-controlled oscillator 6 is significantly greater or significantly smaller than the frequency of the oscillator 1 (more than 1%), the discriminator 5 applies an output signal to the voltage-controlled oscillator in the form of a voltage proportional in amplitude, and corresponding in sign to the difference in the frequencies of the voltage-controlled oscillator 6 and the oscillator 1. This output signal is used to drive the voltage-controlled oscillator 6 nearer to the frequency of oscillator 1. As the frequency of the voltage-controlled oscillator 6 approaches the frequency of oscillator 1, the output of discriminator 5 approaches zero. When the frequency of the voltage-controlled oscillator 6 and the oscillator 1 are very close (usually less than 1%), the phase comparator 2 becomes dominant within the circuit to control the frequency of the voltage-controlled oscillator 6. The phase comparator 2 has inputs 3 and 4 from the reference oscillator 1 and voltage-controlled oscillator 6, respectively, and operates to determine the phase difference between these oscillators. The output of the phase comparator is applied to a low-pass filter 8 which applies a direct current component of the output of the phase comparator 2 to a direct current amplifier 7 for application to the voltage-controlled oscillator 6. This amplified direct current component will drive and hold the voltage-controlled oscillator 6 in phase lock with oscillator 1.
FIG. 2 particularly illustrates the prior art technique used for frequency control of the voltage-controlled oscillator 16. The frequency of the voltage-controlled oscillator 16 is changed by varying the direct current control voltage from D.C. control 18. This control voltage is applied to a reactive element such as variable reactance 17 to change the frequency of the VCO 16 by changing the resonant frequency of tank circuit 13. One such reactive element frequently employed in the prior art is a Varicap. The output from the variable reactance 17 is applied through isolating capacitor 19 to VCO 16 comprising a tuned circuit including D.C. amplifier 11 having a feedback circuit 12 and tuned tank circuit 13 with inductance 14 and capacitance 15. D.C. is equivalent to slowly varying A.C. in this regard.